The present invention relates to an optical disc having an information-retrieval layer for enhanced high signal quality in recording and reproduction.
Optical discs are a storage medium having a transparent plastic support base on which information signals are recorded as minute uneven pit trains or grooves. The information signals are retrieved by emission of laser beams from the surface opposite to the information-recorded surface while intensity of the reflected laser beams is varying in accordance with the recorded information.
This type of optical disc has been widely used, one representative being compact discs (CD) usable at 780 nm-wavelength of recording and retrieving laser beams and 0.45-aperture number of optical-pickup objective lens.
The types of compact disc ranges from read-only type discs (ROM) for information retrieval only, recording type discs (CD-R) to which recording is allowed only once to recording/retrieval type discs (CD-RW) to which recording, retrieval and erasure are allowed several times. Another type is magneto-optical discs having shape similar to these compact discs.
Thanks to reasonable cost for short-wave laser devices and advanced disc-manufacturing technology, the most popular disc is a digital versatile disc (DVD) usable at 635 nm-wavelength of recording and retrieving laser beams and 0.6-aperture number of optical pickup objective lens. High disc density has been achieved thanks to short laser wavelength and high objective-lens aperture number.
Moreover, expected as the next-generation discs are optical discs more dense than DVD, usable at 400 nm-wavelength (or shorter) of recording and retrieving laser beams and 0.7-aperture number (or higher) of optical-pickup objective lens.
The thickness of transparent support base via which information signals are retrieved for optical discs, such as CD, is 1.2 mm usable at 780 nm-wavelength of recording and retrieving laser beams. Formed on the transparent support base is an information-recorded layer such as pit trains and grooves. The information on the information-recorded layer is retrieved from the rear surface. The transparent support base having the information-recorded layer is formed as a transparent retrieval layer.
Optical discs, such as DVD, more dense than CD, have a 0.6 mm-thick transparent information-retrieval support base usable at 635 nm-wavelength of recording and retrieving laser beams. Formed on this transparent support base is an information-recorded layer such as pit trains and grooves. The information on the information-recorded layer is retrieved from the rear surface. The transparent support base having the information-recorded layer is formed as a transparent retrieval layer, like CD. This type of optical discs, however, have a 0.6 mm-disc, called dummy disc for high disc strength.
The next-generation optical discs, usable at 400 nm-wavelength of recording and retrieving laser beams, have an information-recorded layer such as pit trains and grooves formed on a support base having thickness in the range from 1.1 to 1.2 mm. Stuck on the information-recorded layer is a sheet having thickness 0.2 mm or less for recording and retrieval via this sheet surface. One type of next-generation optical disc has such information-recorded layer formed on a transparent retrieval layer.
As discussed, the thickness of optical disc has become thinner as the wavelength of recording and retrieving laser beams has become shorter. In addition, increase in objective-lens aperture number in optical pickup contributes to further high density.
The next-generation discs usable at 400 nm-wavelength of recording and retrieving laser beams and 0.7 (or higher)-aperture number of optical-pickup objective lens usually have thickness of 0.2 mm or less for the information-retrieval layer from the information-recorded surface on which pits or grooves have been formed as information signals to the information-retrieval surface.
Such a thin information-retrieval layer prevents laser beams from diverging during recording and retrieving due to increase in optical aberration such as spherical aberration caused by increase in lens aperture number and coma aberration caused when the disc is bent.
The thinner the information-retrieval layer, however, the more difficult in forming an information-recorded layer on an information-retrieval layer by injection molding like DVD when the support base is 0.6 mm-thick. This is because molding resin will not be filled enough in a molding die by injection molding when a support base is too thin against its diameter, resulting in incomplete formation of information-recorded layer.
To solve such a problem, these next-generation optical discs are manufactured by forming an information-recorded layer on a support base having a thickness of 1.1 mm, for example, by injection molding, followed by forming an information-retrieval layer with an aluminum reflecting film on the information-recorded layer by sputtering, a bonding layer formed on the reflecting film and further a transparent plastic disc sheet having a thickness of 0.1 mm, for example, on the bonding layer.
Evaluation tests revealed, however, that these next-generation optical discs having a 0.1 mm-thick transparent plastic disc as the information-retrieval layer suffer from large variation in reproduced waveforms per one disc rotation. It is further revealed that one cycle of variation in reproduced waveforms mostly has two-cycle components.
An automatic gain controller suppresses such variation a little bit, but still not enough against asymmetry and degraded frequency characteristics of the varied output waveforms, thus providing no constant output.
Several sample discs suffered from peak-to-peak variation in reproduced waveforms, some reproducing output almost half of the minimum level.
The variation in reproduced output peculiar to the next generation optical discs having such thin information-retrieval layer cause increase in output errors, an inevitable halt to normal retrieval operation.